1. Field of the Invention
The present invention relates to an anti-skid brake system for an automotive vehicle so adapted as to control excessive braking force at the time of braking the vehicle and, more particularly, to improvements in the technology for estimating a .ground friction status of slippery ground on which the vehicle is running.
2. Description of the Related Art
As a braking system for braking an automotive vehicle, an anti-skid brake system is commercialized which can prevent wheels from locking or skidding at the time of braking.
The anti-skid brake system of this type has wheel speed sensors for sensing wheel speeds of the four wheels, an electromagnetic control valve for adjusting braking oil pressure, and a control unit for controlling the electromagnetic control valve on the basis of the wheel speeds sensed by the wheel speed sensors. The control unit is arranged such that, for example, the acceleration or deceleration of the wheels is determined on the basis of the wheel speeds of the wheels sensed by the wheel speed sensors and that the braking force is decreased by controlling the electromagnetic control valve when the deceleration is below a predetermined value (a braking force-decreasing phase), while the braking force is increased by controlling the electromagnetic control valve so as to increase its braking force when the wheel speeds of the wheels are increased by decreasing the braking force and when the acceleration reaches a predetermined value (a braking force-increasing phase).
If a series of the operations for controlling the braking force (referred to sometimes as "ABS control") is continued, for example, until the vehicle halts, it can prevent the wheels from being locked or skidding during rapid braking and the vehicle can be suspended in a short braking distance while ensuring a dimensional stability of the vehicle body.
The ABS control is usually carried out by estimating a vehicle speed from the wheel speeds, estimating a friction coefficient .mu. on slippery ground on which the vehicle is running on the basis of the rate of change of the vehicle speed, and correcting a threshold value for shifting the braking force between the braking-force decreasing phase and the braking-force increasing phase on the basis of the resulting ground friction coefficient. Improvements in accuracy of estimating the vehicle speed and the ground friction coefficient are extremely significant for improving the performance of the anti-skid brake control. For example, Japanese Patent Unexamined Publication Kokai No. 61-222,854 proposes that the ground friction coefficient is estimated on the basis of the maximal wheel speed out of the wheel speeds of the four wheels.
Such a conventional anti-skid brake system, however, is not structured such that the ground friction coefficient is estimated by taking account of the fact that the wheel is locked when it slips to an extent exceeding a predetermined value. As the number of the locked wheels has a close relationship with the ground friction coefficient in such a manner that the number of the locked wheels is increased when the ground friction coefficient becomes low,-while the number of the locked wheels is decreased when the ground friction coefficient becomes high. Hence, there is still room for improvements in accuracy of estimating the ground friction coefficient for such an anti-skid brake system.